This invention relates to a hydraulic actuator and more particularly to a dual stage hydraulic actuator for operation of an expanding gate valve.
Heretofore, the use of fluid-powered actuators to effect the operation of gate valves has been generally restricted to use with gate valves of the slab-gate type. The very large sealing forces of valves of the expanding gate type require a very powerful actuator thrust to break the seal and such a powerful thrust runs a high risk of causing serious damage to the valve components or the actuator itself. Also, very large piston springs are required to return the actuator piston upon release of fluid pressure from the actuator and such springs so significantly increase the size, weight and expense of the actuator that their use is practically prohibitive. Nevertheless, the inherent advantages provided by expanding gate valves in the handling of very high fluid pressures has created a need for the use of fluid-powered actuators with expanding gate valves.
The use of a pneumatic-powered actuator for opening and closing an expanding gate valve is shown in U.S. patent application Ser. No. 568,460, filed Jan. 5, 1984, now U.S. Pat. No. 4,535,967. However, hydraulic actuators as opposed to pneumatic actuators are preferred in many applications, such as subsea environments, where power, small size and reliability are important criteria. The use of hydraulic actuators for the operation of expanding gate valves presents many problems. The very large thrust force which is required to operate these valves when in the expanded open or closed conditions, will break the oftentimes crush the actuator stem or deform it such that the valve or actuator are so damaged as to become inoperative. This probably cannot be readily solved by providing a stronger, larger actuator stem because the associated required increase in the size of the of actuator becomes prohibitive.
Most hydraulic actuators used with slab-type gate valves utilize a sleeve or piston downstop which is usually installed between the piston and the bonnet adapter plate to prevent the application of the maximum actuator load to the valve stem but because of the tolerances involved in the gate wedging geometry, it is not practical to install a piston downstop for actuators for expanding gate valves.
One possible solution is to simply derate the actuator to allow it to function at approximately 11/2 times its maximum operating pressure. Because of the wide variation in supply pressures for hydraulic systems in well production systems, generally 1500 psi and 3000 psi systems, it is not popular or practical to simply derate the actuator to allow it to function at only 11/2 times its minimum operating pressure.
Accordingly, it is an object of the invention to provide a hydraulic actuator which will deliver the required output load to operate an expanding gate valve yet still be able to accept very high actuating fluid pressures without overstressing the actuator stem or other valve components.
A further object is to provide a dual stage fluid-powered actuator of the piston type which will provide a very high initial thrust in its first stage of operation and a reduced thrust in the latter stage of the actuator stroke.